There are a number of appliances such as cooking ranges and clothes dryers and heating apparatuses such as boilers and furnaces in which a combustible material, such as a combustible hydrocarbon (e.g., propane, natural gas, oil) is mixed with air (i.e., oxygen) and continuously combusted within the appliance or heating apparatus so as to provide a continuous source of heat energy. This continuous source of heat energy is used for example to cook food, heat water to supply a source of running hot water and heat air or water to heat a structure such as a house.
Because this mixture of fuel and air (i.e., fuel/air mixture) does not self-ignite when mixed together, an ignition source must be provided to initiate the combustion process and to continue operating until the combustion process is self-sustaining. In the not too distant past, the ignition source was what was commonly referred to as a pilot light in which a very small quantity of the combustible material and air was mixed and continuously combusted even while the heating apparatus or appliance was not in operation. For a number of reasons, the use of a pilot light as an ignition source was done away with and an igniter used instead.
An igniter is a device that creates the conditions required for ignition of the fuel/air mixture on demand, including spark-type igniters such as piezoelectric igniters and hot surface-type igniters such as silicon carbide hot surface igniters. Spark-type igniters that produce an electrical spark that ignites gas, advantageously provide very rapid ignition, which is to say, ignition within a few seconds. Problems with spark-type igniters, however, include among other things the electronic and physical noise produced by the spark.
With hot surface igniters, such as the silicon carbide hot surface igniter, the heating tip or element is resistively heated by electricity to the temperature required for the ignition of the fuel/air mixture, thus when the fuel/air mixture flows proximal to the igniter it is ignited. This process is repeated as and when needed to meet the particular operating requirements for the heating apparatus/appliance. Hot-surface-type igniters are advantageous in that they produce negligible noise in comparison to spark-type igniters. Hot surface-type igniters, however, can require significant ignition/warm-up time to resistively heat the resistance igniter sufficiently to a temperature that will ignite gas. In some applications, this warm-up time can vary between about 15 and about 45 seconds.
In recent years, efforts have been made to develop a robust, low-noise igniter that can ignite gas rapidly, which is to say within a few seconds. There is found in U.S. Pat. No. 4,925,386 a control system for electrical resistance-type igniters, and more specifically for tungsten heater elements embedded in a silicon nitride insulator. The relatively narrow temperature operating range of silicon nitride igniters necessitates such a control system. Indeed, the operating range of silicon nitride igniters must remain between the lowest temperature that will ignite gas and the temperature at which the igniter fails, i.e., the tungsten heater element breaks down.
Over time, this narrow range of operating temperatures is further narrowed due to a process referred to as “aging”. As the tungsten heater elements are repeatedly heated to relatively high temperatures, the tungsten filaments oxidize or “age”. Aging manifests as a cross-sectional change, i.e., decrease, in the tungsten filament. As a result, acceptable operating temperatures routinely decrease and continue to decrease with further aging. The described control system includes a microprocessor and a learning routine to control and modulate a solid-state switching means so that the igniter can be heated rapidly to and maintained at or near a suitable ignition temperature, which is below the maximum operating temperature. Moreover, the described learning routine maintains the temperature of the igniter just above the temperature needed to ignite the gas, to provide quick ignition, while continuously monitoring the maximum allowable temperature to prevent damage to the igniter.
Similarly, there is found in U.S. Pat. No. 5,725,368 a refined control system that controls the energizing of a silicon nitride igniter that, purportedly, enables ignition within approximately two seconds. The described control system includes a microcomputer in combination with a triac in series with an igniter and a learning routine. The microcomputer determines the level of power to be applied to the igniter as a function of the voltage available to energize the igniter and the resistance of the igniter. The triac delivers time-dependent power to the igniter using an irregular firing sequence.
There are, however, several shortcomings with these two control systems. First, they are drawn to a specific igniter type that is subject to “aging”. As a result, the systems require hardware and software to enable the learning routine. They also continuously maintain the temperature of the igniter slightly above the minimum ignition temperature, e.g., about 1200 degrees Centigrade. Thus, it would be desirable to provide a robust control system for energizing a hot surface-type igniter of a type that is not susceptible to significant aging and does not have to maintain the igniter continuously at about 1200 degrees Centigrade.